An internal multiple is an event in recorded seismic data that has been reflected off at least two subsurface reflectors. Because most imaging algorithms have issues with multiple reflections, also called multiple scattering or internal scattering, internal multiples are traditionally considered noise that needs to be removed from the data (see e.g. Jakubowicz, 1998 or Verschuur and Berkhout, 1996). An internal multiple is to be distinguished from a surface-related multiple, which is a seismic event that includes at least one reflection from the air interface in addition to at least two reflections from subsurface reflectors. However, the multiple reflections do contain some information about the subsurface, and recently there has been a movement to capture that with a method called full-wavefield inversion (“FWI”), in which the entire wavefield of seismic data, including the multiple reflections and the direct transmissions from source to receiver, may be inverted to infer a physical properties model, such as a velocity model of the subsurface.
Redatuming of seismic data is a process in which, based on the measured data, one calculates the data that would have been measured if the sources and/or the receivers had been placed in a different location. An insightful example of redatuming is the correction applied to data measured in an environment where the surface is corrugated. Imaging algorithms usually do not deal very well with data that comes from sources and receivers at different altitude, and therefore, a new datum is chosen, usually below the elevation of any of the actual sources and receivers. The new datum will be populated with virtual sources and virtual receivers. The data that these virtual receivers would measure is calculated from the actual measured data. This could be done in a way that corrects the travel time of the seismic wave between the virtual source on the new datum and the actual source plus the travel time between the virtual receiver on the new datum and the actual receiver. By doing so, one tries to remove the influence of the subsurface above the new datum. Of course events that reflect once in the subsurface above the new datum will appear at non-causal times after the corrections, and one can therefore identify them as not coming from the subsurface below the new datum and discard them. However some internal multiples will reflect in the subsurface above the reflector many times, and they will not appear at non causal times after the correction. They may erroneously be seen as events coming from reflections from below the new datum. This error can lead to an incorrect interpretation of the subsurface and lead to drilling dry wells or missing a lucrative oil and gas reservoir. This is an example of a technical problem that the present invention can address.
In seismic surveying, illuminate is a term of art meaning how well an object in the subsurface can be detected by seismic waves. It might be physically impossible to send a seismic wave that is going to reflect at all parts of the object, simply because we are usually able to put sources only at the surface. For the same reason, it might be physically impossible to receive all reflected seismic waves from all parts of the object. The more parts of the object that can be detected, the higher the illumination of an object.
Previous attempts to remove internal multiples from seismic data include the following.
U.S. Pat. No. 7,987,054, “Efficient multiple prediction in two and three dimensions,” to Baumstein; US Patent Application Publication No. 2012/0041682, “Attenuating internal multiples from seismic data,” by Ramirez-Perez et al.; and US Patent Application Publication No. 2011/0199858, “Estimating internal multiples in seismic data,” by Otnes et al., are all recursive internal multiple prediction methods. As such, their method must be applied several times, each time addressing a small group of reflectors in order to remove all the internal multiples at a desired depth. The present inventive method needs to be used only once to remove all internal multiples at a desired depth.
US Patent Application Publication No. 2010/0135114, “Wavefield extrapolation modeling for internal multiple prediction,” by Teague et al. is a wavefield extrapolation method and, as such, is a model-driven and not a data-driven method. As such, it needs to model data based on a spatial model of the subsurface that includes at least the location of the strongest reflectors. The present inventive method does not need a spatial model of the subsurface.